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[Purr Data as a Web App](#inject-purr-data-directly-into-the-web)
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[Profile and Optimize Purr Data for Realtime Safety](#profile-and-optimize-purr-data-for-realtime-safety)
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[Terminal REPL](#terminal-repl)
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[Core Accessibility](#core-accessibility)
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[Purr Data Message and DSP Profiler](#purr-data-message-and-dsp-profiler)
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[Streamlining Purr Data GUI-Pd communication](#streamlining-purr-data-gui-pd-communication)

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[Vintage Platform Audio Emulation Library](#vintage-platform-audio-emulation-library)

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[Library for Data-Over-Audio Communication](#data-over-audio-messaging)
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[Interaction with Audio Plugins](#interaction-with-audio-plugins)
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[JIT-compiled signal graph for the audio engine](#jit-compiled-signal-graph-for-the-audio-engine)
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[Use ref-counting to handle object lifetimes](#use-ref-counting-to-handle-object-lifetimes)
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[Visual Diff](#visual-diff)
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[Encapsulation Ergonomics](#encapsulation-ergonomics)

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[Fake News Audio Library](#fake-news-audio-library)

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[Worst of All Possible Worlds Interpreter](#worst-of-all-possible-worlds-interpreter)

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Inject Purr Data Directly into the Web
--------------------------------------
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### Goal
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Add a WebAssembly target and pure HTML5 GUI framework for running Purr
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Data as a web app.
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### Benefits
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Installing Purr Data is often the biggest hurdle for newcomers. Running
it in a web browser simplifies this process and allows Purr Data to be run
on any device, with any architecture, as long as it has a web browser installed.
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### Details
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For the core, we want to produce a WebAssembly binary that can load a patch
and produce audio using the Web Audio API. If possible, we want to also
leverage Chromium's newer callback-based interface to compute realtime audio
as efficiently as possible.

For the GUI, we need a framework that can display and edit a Pd patch
within a container in a web page. It should be possible to run the core
WebAssembly binary with or without the GUI displayed.

The challenge with the GUI is that Purr Data currently uses multiple
toplevel windows to display a patch and its visible subcanvases, and
properties windows for certain widgets. The framework needs to allow
viewing all of these in a container within a *single* browser window.
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### Challenges
There has been some prior art for building a WebAssembly binary
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for the core audio engine. However, the difficulty will lie in building
the GUI framework that has a workable UX within a single browser window.
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### Languages
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C, HTML5, and Emscripten
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Profile and Optimize Purr Data for Realtime Safety
--------------------------------------------------

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### Goal
Find the "pain points" in Purr Data's core message dispatcher and
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audio engine, and optimize the code where possible to improve the realtime
scheduling of DSP.

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### Details
The core DSP algorithms of Purr Data tend to avoid system calls,
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unnecessary branching, and other calls which would make the performance of
the audio process unreliable. However, there are many areas of Purr Data
which are not optimized for realtime scheduling.

The first order of business would be to profile Purr Data in several areas
to see where problems may be. Some likely culprits are the following:

* parse time for incoming messages from GUI to the audio/messaging engine
* overhead of sending messages to the GUI, especially for visual arrays and
  drawing instructions for data structures
* overhead of sending streams of "motion" messages from GUI to Pd to track
  mouse position
* overhead of walking the linked list of objects in order to find the object
  under the mouse/mouseclick
* overhead of tracking mouse position over a visual array
* overhead of calculating bboxes in the audio process as opposed to GUI
* overhead of calculating text positioning in audio process as opposed to GUI
* overhead of counting UTF-8 code points in audio process to calculate
  line breaks
* overhead of handling incoming/outgoing socket traffic in the same thread
  as the audio scheduler
* probably other areas

Once we get a sense of the pain points, we can tackle the problem of how
to optimize in a maintainable manner. Some references for this:

* [Guilio Moro's work](https://github.com/giuliomoro/purrdata/commit/9dc3223ece79be5f60a6a629450b52a79b9e050c) on using a separate thread for the socket connections from GUI to the core audio engine (plus all the other socket connections
like netsend, netreceive, etc.)
* Guilio Moro's work on a threaded microsleep for the event loop.
* [Guilio Moro's work](https://github.com/giuliomoro/purrdata/commits/simpler-motion) to simplify GUI communication by handling more of the mouse motion/click
  logic in the GUI. This results in fewer messages from GUI to audio engine,
  but still requires a linked-list walk in the audio engine to find the
  relevant object.
* Matt Barber's link to a new cosine wave generator algorithm that may be
  more performant than the current implementation. (Not so important for
  current performance, but this may become more relevant once we switch to
  double-precision for block samples.)
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* Possibility to vectorize DSP algos using SIMD. Also more crude experiments
  by just hand-unrolling one or two classes when N=64 (i.e., the most common
  block size) and measuring the performance impact (if any).
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Note: There may be some overlap with the other profiling idea listed below.
Developers for both ideas may therefore benefit by periodically sharing their
work with each other.

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### Challenges
The initial profiling will take some time but
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isn't particularly challenging. Making changes to the core audio engine,
however, will require some knowledge of Linux system interfaces and some of
Purr Data's internals. Properly assessing and testing any threading techniques
in C is also frought with peril and will require extreme care in order to
keep the code maintainable and avoid insidious bugs.

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### Languages
Basic to advanced shell scripting, C, plus familiarity with
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profiling tools like gprof and others.

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Terminal REPL
-------------

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### Goal
Make a little REPL interface with which the user can interact with
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Purr Data programs and program state.

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### Details
Purr Data is being used in situations where the hardware is an embedded
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device. While the current GUI runs on most common hardware including the RPI,
there are situations where it would be more convenient to simply interact using
a text interface (locally or over ssh).

The user can already communicate with Purr Data's audio engine over a socket
connection. So the "read" and "evaluate" part already exists. However, Purr
Data does not print a response nor loop in this situation.

Additionally, the UX of sending raw messages to Purr Data's interpreter is
quite lacking. The syntax for creating new environments and objects was not
meant to be used directly. Objects are referenced by index number, and the
diagrams themselves must be referenced using hex identifiers.

It would be very beneficial to create a REPL UI that is more user-friendly
and well-documented/specified. This way Purr Data users can always interact
with and create programs easily on any embedded device, even if there is
no direct display. (This would also be very handy for debugging purposes.)
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### Challenges
An initial REPL can be created with the current Purr
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Data API, but it won't be particularly user-friendly. To achieve that requires
more work and an understanding of Purr Data's message dispatching system.

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### Languages
C, some shell scripting.
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Core Accessibility
------------------

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### Goal
Ensure that Purr Data is accessible by coupling accessibility
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with the core UX

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### Details
Especially because Purr Data is a graphical environment, it's
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important to make sure the core functionality is accessible. Rather than
tack on accessibility as an afterthought, Purr Data should have a UX that
makes accessibility features a generally useful part of the programming
environment.

For example: how does one navigate the nodes of a Purr Data diagram? There
should be a way to navigate among the nodes and their connections
without using the mouse. If we make sure that each element in the diagram
is annotated we can tackle accessibility and keyboard navigation at the
same time. Thus, a robust keyboard navigation implementation will help
make it possible for screen readers to give meaningful information about
each node in the graph.

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Note: there may be some overlap with the REPL idea above, as the REPL could
provide a sensible way for a user to traverse the diagram as an alternative
to using the GUI.

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### Challenges
For example, it will be necessary to study the current
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GUI implementation to figure out how to extend it to add keyboard navigation.
It will also be necessary to study pre-existing approaches to making SVG
diagrams accessible and study the current state of HTML5 tools that facilitate
this.

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### Languages
Javascript, HTML, CSS. Some basic C knowledge may be required to
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send a richer set of data about each object from the core to the GUI. However,
there is already an interface that can do this-- it just needs to be hooked
into the GUI.

Purr Data Message and DSP Profiler
----------------------------------

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### Goal
Measure the time it takes for each object in a Purr Data diagram to
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process its data and display the results in the diagram.

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### Details
Purr Data users would benefit greatly from the ability to profile
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their programs while they are running. This is easy to do for the program
as a whole, but challenging to do per-object.

A successful implementation of this feature will give an accurate measure
of the time it takes each object to process its incoming data. This needs to
support all of Purr Data's platforms: Windows, OSX, and GNU/Linux.

A successful implementation will also be performant enough that the
measurements themselves don't impact the realtime operation of Purr Data
itself.

A now defunct fork of Pure Data called ["DesireData"](http://artengine.ca/desiredata/)
did an initial
implementation of this idea using the x86 RDTSC instruction. (Though its
unlikely this feature was actually stable at the time DesireData was
in active development.) Though this instruction is no longer considered
reliable on modern machines, the overall approach taken by DesireData of
adding a field to the t_gobj struct for storing this timing data is
probably a sound starting point.

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Note: There may be overlap with the other profiling idea listed above, as
developers on both ideas will probably be using the same tools and can
therefore benefit by periodically sharing their work with each other.

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### Bonus goal
Figure out a way to meaningfully profile DSP objects. DSP objects
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typically process data at a high sample rate (44,100 is common) so displaying
the data in a user-friendly and meaningful way is tricky.

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### Challenges
This feature touches the main artery of the
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message dispatching system, and the bonus goal would touch the main DSP
routine. In both cases realtime scheduling deadlines must be taken into
account by careful profiling.

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### Languages
C for the profiling business logic, HTML5 for displaying the
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results in the GUI.
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Streamlining Purr Data GUI-Pd communication
------------------------------------------

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### Goal
Move some of the GUI callbacks out of Purr Data's audio engine so that GUI
interaction is less likely to cause dropouts.

### Details
The Pd GUI is heavily entangled with the Pd audio backend. In fact, most of the
"gestures" performed on the GUI are passed straight to the Pd engine for
processing. The GUI gestures are then "analyzed" by the audio thread, which may
respond with triggering a GUI action, changing the state of an object, or
nothing.

For instance, each mouse move triggers a `motion` message to the Pd backend,
handled by `canvas_motion()` in `g_editor.h`. This calls
`canvas_doclick(... doit = false)`, which in turn iterates through all the
objects on the patch and asks each of them "does the cursor happen to be on top
of you?" (`canvas_findhitbox()`/`canvas_hitbox()`), calling a callback function
(`w_getrectfn()`) for each of those objects.

Now, most of the time the cursor is not on an object (or patch cable) and the
calls to `w_getrecfn()` have no effect, except for wasting CPU power. There are
two notable exceptions: 
a) when the mouse pointer is on top of an object, or one of its inlets or
outlets, or on top of a patch cord, or on top of a GUI object, the mouse
pointer may change, plus, e.g.: flickering inlets/outlets.
b) some objects use the calls to `w_getrecfn()` to track mouse position
(e.g.: [mousestate] from cyclone).

The above results in a plethora of CPU cycles being wasted, which may cause
dropouts when using small blocksizes and/or embedded platforms. Besides - and
perhaps most importantly - it seems the wrong approach that some GUI-specific
actions (like the ones at a) above) have to be processed and validated by the
audio engine, within the audio thread.

We could therefore think of an improvement to the Purr-data architecture, where
the GUI stuff (e.g.: point a) above) is delegated uniquely to the GUI, which
makes for lower CPU usage and potentially a more responsive GUI. For instance,
the GUI could be designed to only send `motion` messages when the mouse is on
top of an object and it could send alongside with it the Pd "tag" of the object,
so that `w_getrectfn()`  can be called only for the relevant object).

The optimal approach would involve handling all the graphics effects (in/outlet
animation, mouse pointers) directly within the GUI, and only sending `motion`
messages when something relevant to the Pd engine is _actually_ happening
(e.g.: when connecting objects).

Additionally, and looking forward, in order to address point b), objects that
need to track mouse position should declare this at initialization and should
be kept in a dedicated list, so that the `motion` messages from the GUI can be
delivered only to them with minimal CPU waste.

An alternative - and probably worse - approach to the problem, which could
reduce peak CPU usage, would be for the Pd audio engine to maintain a
"rasterized" cached map of the patch (e.g.: by calling `w_getrecfn()` for each
object at each pixel). This way, it could simply look up the cached map in
response to each `motion` message. The cache could be recomputed in a separate
thread every time after a new object or patch cord is created. Threading issues
may arise here, in case one of the objects is deleted while the cached map is
being built.

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### Challenges
This project comes with a number of
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challenges, including: potential threading issues between the engine and the
GUI, the necessity to re-write the C code of some objects, providing complete
documentation for creators of externals, maintaining - where possible (e.g.:
excluding objects that track mouse position) - backwards compatibility with Pd.

More details on a previous attempt at addressing the problem can be found
[here]( http://disis.music.vt.edu/pipermail/l2ork-dev/2017-June/001383.html).

### Languages
Javascript and C.
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### Potential Mentor
Giulio Moro

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Vintage Platform Audio Emulation Library
----------------------------------------

### Goal
Create a library with objects that emulate the hardware from
old hardware like the atari 2600, NES, and others.

### Details
There are a lot of resources online for emulating old
hardware. Purr Data would benefit by having a library
that provides a consistent interface for objects that
take input into an emulation of a piece of hardware and
output one or more audio signals.

If possible, it would be beneficial if most of the interface
could be built as a set of abstractions. That way more
developers would be able to understand and improve the library.

There is a TIA chip emulator written in C in externals/mmonoplayer
that can be used as a starting point.

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### Challenges
Finding a common interface that makes it easy for users to leverage
these classes while at the same time being expressive enough to allow
decent control of the chip being emulated.
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### Languages
Pd (Purr Data is a fork of the software Pure Data-- the visual language itself
is usually referred to as Pd.) Also, C.

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Data Over Audio Messaging
-------------------------

### Goal
Create a library that allows two instances of Purr Data to pass data messages
to each other using sound as the transmission medium.

### Details
Pd messages consist mainly of space-separated numbers and symbols.
Semicolons mark the end of a message.

Sometimes it would be helpful to be able to pass messages from
one instance of Purr Data to another-- especially if each instance is
on a different machine in the same room. This is currently done either
by setting up socket listener/receiver between the two instances or by
leveraging a separate message-passing system outside of Purr Data.

Since Purr Data is concerned mainly with analyzing and sythesizing sound,
machines running Purr Data typically have a mic and speakers connected to
a running instance. If it were possible for the user to simply create
objects which send/receive messages by sending audio signals to/from each
other it would greatly simplify sending at least small amounts of data
between machines.

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### Challenges
Finding a decent interface for users without relying on a big set
of dependencies.
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### Languages
Pd (Purr Data is a fork of the software Pure Data-- the visual language itself
is usually referred to as Pd.) However, the library may also be written in
C.
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Interaction with Audio Plugins
------------------------------

### Goal
Make sure that Purr Data has a well-documented way to accept input from 
standard audio plugin APIs like VST, LADSP, and LV2. Also
make sure Purr Data can be used as a plugin in other environments.


### Details
There are multiple audio plugin APIs that aim at seamlessly mixing and
matching audio filters, synthesizers, and analysis tools in different
languages and applications.

Purr Data has some libraries to interface with at least two of these
standards (VST and LADSPA). There is also a library for LV2 that
Purr Data can leverage. However, not all of these libraries run on
all the supported platforms (OSX, Windows, Linux).

Purr Data also has all the APIs necessary to act as a plugin itself
in other applications. But work must be done to ensure this works
properly and that it is properly documented how to do it.

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### Challenges
There is a lot of pre-existing technology 
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here, but it needs to be tested rigorously on all platforms.

### Languages
C. Also, familiarity with shell scripting as well as Gnu make.

JIT-compiled Signal Graph for the Audio Engine
----------------------------------------------

### Goal
Leverage LLVM to make a jit-compiler for Purr Data's DSP graph.

### Details
Alex Norman has shown that it is possible to build a jit compiler
for one of Pd's "workhorse" libraries-- the "expr" library. This
library essentially lets users specify a mathematical expression
that can operate on both vectors and on the sample level. The
current library parses the tokens of the expression and requires
a separate function call for each unit. This is similar to the
way Pd's DSP graph itself gets executed.

The jit compiler produces assembly which has superior performance
to the current "expr" library. Extending that process to the entire
signal graph (or at least core classes within it) would benefit
performance on a greater level.

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### Challenges
Finding a way to incrementally build up the functionality
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Since there is pre-existing
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work done with the jit "expr" library, a good start would be to help
test that library and improve it. Once the student gains a mental
model of the process the student can begin to extend it to Purr Data's
DSP graph itself.

### Languages
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C++. Knowledge of LLVM will also come in handy here.
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Use ref-counting to handle object lifetimes
-------------------------------------------

### Goal
Use ref-counting to track object lifetimes in a more robust manner.

### Details
Currently there are many places in the message dispatching system where an
object may disappear before a message bound for it gets dispatched. This,
along with dynamic patching and arbitrary patch loading can easily cause
Purr Data to crash.

Additionally, this limits the possibility for reporting errors from within
a given call stack. Currently we have no way to know if any of the objects
participating in a given call still exist at the time of an error. This
means we're limited to checking after the fact for any signs of mutation
at error time, and then simply refusing to trace in that case. That prevents
valuable analytical data for the most complex error cases.

### Challenges
There aren't currently any tests for the cases where attempting to
dereference a pointer to a freed object causes a crash. Also, there are
two "stacks"-- one is the messaging system proper. The other is the
loading mechanism used to open a file or abstraction.

Additionally, Purr Data has a binding system which can dynamically bind
or unbind symbols even within a single call stack.

### Languages
C.
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Visual Diff
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-----------

### Goal
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Make a tool to visually show the changes in a Purr Data patch, possibly
with animation.
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### Details
Pd has a source file format that is difficult to read. Furthermore,
changes in z-order in the GUI can make changes in the source nearly
impossible to convey. This makes it difficult to glean anything of
substance by viewing a diff of a Pd file in git.


### Challenges
This is a general problem with visual programming languages. There may
be some prior art with some languages that attempt to solve the problem.
But unlike Pd, those languages were probably built from the ground up
to solve that problem, whereas Pd's file format doesn't lend itself to
solving that problem.

### Languages
C, possibly HTML5.
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Encapsulation Ergonomics
------------------------

### Goal
Improve Purr Data's techniques for encapsulation in the language.

### Details
Pd currently has two basic techniques for organizing diagrams. One is simply
to copy part of the diagram and paste it into a subpatch. That part of the
diagram becomes hidden inside a new object that appears on the parent window.

The other is to use so-called "abstractions", which are other Pd files that
are instantiated as an object on the parent.

Both cases present problems for developers. For example, the program should
be able to take a selection of objects and immediately put it into a subpatch
with the relevant inlets and outlets created inside the subpatch. Instead,
the user must manually cut, paste, and redraw the connections in the editor.

Second, users who want to encapsulate and reuse code must first save a
patch to the filesystem in the current Pd path, and then create a new object
with the same name as that file (minus the ".pd" extension). This makes
proper encapsulation more difficult than simply cutting/pasting subpatches,
leading to quick-and-dirty copy/pasta where more robust code reuse through
abstractions would have been preferred.

### Challenges
Much of Pd's current code assumes that abstractions must be loaded from
the file system instead of from some template that already exists in the
current patch file.

Additionally, a new class must be added to define a patch template that
fills the role that ".pd" files currently do for abstractions.
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Fake News Audio Library
-----------------------

### Goal
Create a set of DSP filters which take common misconceptions about digital
audio and one or more inputs to generate an output that makes those
misconceptions true.

### Details
There is a plethora of misconceptions about digital audio running back
decades. Everything from the wildly subjective (e.g., compact discs don't sound
as "warm" as vinyl records), to the technical (e.g., as the frequency
in a digital audio signal approaches Nyquist the accuracy of that signal
degrades).

Once a workable list of such misconceptions is enumerated and cross-referenced,
a library should be created for each to take an arbitrary signal input and
generate an output in line with that particular misconception. For example,
if the misconception is "digital audio degrades higher frequencies", the
higher frequencies in the input should be degraded in the generated output
signal.

### Challenges
The "misconceived" output for each library must be predictable so that
a potential user of the library is able to reason about the features of that
library. It must also be sophisticated enough that one who believes the
misconception doesn't reject the filter out-of-hand as an unfair
exaggeration of their erroneous belief.
Jonathan Wilkes's avatar
Jonathan Wilkes committed
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Worst of All Possible Worlds Interpreter
----------------------------------------

### Goal
Write a small scripting language for Purr Data where the time it takes to
compute the output is always the worst case time.

### Challenges
BPFe rules here-- user can write loops but not loop indefinitely. User may have conditionals. No unsafe wizardry allowed.

### Bonus Challenge
Allow user to specify asyncronous behavior with a single float or int value.

E.g., default of 1 means that input will trigger the entire script to run and
trigger the output.

0.5 means input will trigger the script to run *half* its operations and return
control. A subsequent input will run the second half of the operations and
trigger the output.

0.25 means input will trigger a quarter of the operations to run.

Etc.